Thermoelectric conversion materials are used for utilization of waste heat by converting waste heat energy to electric energy employing Seebeck effect or Peltier effect.
For example, as described in Patent Literature 1, micro single crystals of such as NaCo2O4 having a layered perovskite structure have significantly better thermoelectric properties in a-axis direction compared with those of Bi2Te3 system, which are representatives of conventional metal-based thermoelectric conversion materials. Accordingly, as Patent Literature 2 and Non Patent Literature 1, attempts are made to produce large single crystals of NaCo2O4, but production of the single crystals usable in thermoelectric generation is difficult and costly and is not achieved.
For this reason, many researchers have attempted to produce polycrystalline bodies of NaCo2O4 which exhibit high efficiency. However, only significantly lower efficiency compared with that of the single crystals is achieved in a current status. This is because the efficiency of the single crystal in c-axis direction (stacking direction of NaCo2O4, which is orthogonal to a-axis direction) is about 1/200 of that in a-axis direction, and polycrystallization brings about only an average efficiency at best. This means that in order to provide polycrystals with higher efficiency, it is necessary to direct all the crystals toward high-efficient direction.
In general, polycrystalline NaCo2O4 is produced by preparing a mixture of powders of Na2CO3 and a Co oxide such as Co3O4, and heating (sintering) the mixture in a temperature range of 880 to 950° C. Here, the powders tend to be micronized so that reaction progresses uniformly and rapidly.
However, such a production method suffers from a drawback that the efficiency of NaCo2O4 is deteriorated due to other factors besides the anisotropy of crystal orientation, such as cavities caused by gas generated through the reaction, existence of unreacted portions and heterogeneous phases, and weak bonding at grain boundaries. Here, for the purpose of suppression of the other factors such as cavities, methods such as a hot press method and a plasma sintering method is considered and a certain level of achievement is made. However, these methods have no effect on control of the orientation.
For this reason, the efficiency of the polycrystalline thermoelectric conversion materials is extremely poor compared with that of the single crystals.
Moreover, as an attempt to obtain oriented polycrystals, there is a method in which plate-like particles of cobalt hydroxide or cobalt oxide and sodium metal salt are mixed, the mixture is formed in such a way that the plate-like particles are oriented in one direction, and the compact thus obtained is sintered to be densified, thereby producing a sintered body having c-axis direction oriented toward one direction.
In Patent Literature 3 and Non Patent Literature 3 employing this method, control of the orientation toward <001> direction is achieved at a ratio of 90%, which indicates that the efficiency is improved by several times as compared with that of a non-oriented sample.
As research aimed to control the orientation in thin films, for example, in Non Patent Literatures 3 and 4, control of orientation in Ca3Co4O9 and SrCo4O9, which are substances similar to NaCo2O4, is achieved.